The dye-sensitized photooxygenation of p-hydroxyphenylpyruvic acid (1) to a quinol intermediate which, on raising the pH is converted to homogentesic acid (2) has been studied in detail as a possible model for the biological conversion of 1 to 2. It was found, however, that the mechanism of the enzymic conversion of 1 to 2 differs from that of the photooxygenation.